只有在完成所有同步請求後才發出信令通知

Question

當所有異步請求都完成時收到通知。

我在下面的循環中多次呼叫registerTrack。我只想觸發syncGroup.notify時全部請求已完成。從看這個代碼，在registerTrack每次成功完成之後，它會去notify。只有在所有registerTrack操作完成後，我如何才能使其顯示notify？

我的代碼如下：

var fail = false 
    let syncGroup = DispatchGroup() //used so we can determine if all the register requests have finished 

    for track in unsyncedTracks { 
     syncGroup.enter() 

     if fail { 
      syncGroup.leave() 
      break //might save an aync request from getting executed on failure 
     } 

     registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

      if success { 
       self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
       syncGroup.leave() 
      } 
      else { 
       fail = true 
       syncGroup.leave() 
      } 
     }) 
    } 

    //all requests complete 
    syncGroup.notify(queue: .main) { 
     if fail { 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

編輯：

基於使用GCD併發隊列的建議，我改變了我的代碼：

var failed = false 
    //work item to loop through and call registerTrack on each track 
    let registerTracksWorkItem = DispatchWorkItem { 
     for track in unsyncedTracks { 
      if failed { //exit early on failure, potentially save a network request 
       break 
      } 

      self.registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

       if success { 
        self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
       } 
       else { 
        failed = true 
       } 
      }) 
     } 
    } 

    //handler for when all registerTrack calls are complete 
    registerTracksWorkItem.notify(queue: DispatchQueue.main) { 
     if failed { 
      self.debug.log(tag: "RecordViewController", content: "At least one registerTrack call failed") 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

    //execute the work item 
    let queue = DispatchQueue.global() 
    queue.async(execute: registerTracksWorkItem) 

此代碼不等待registerTrack完成，而是執行它們並調用notify事件。 :(

編輯3：

所以這個作品它使用一個計數器進行檢查，看其是否在最後unsyncedTracks對象

var fail = false 
    let syncGroup = DispatchGroup() //used so we can determine if all the register requests have finished 

    //used to make sure notify is only triggered when all registerTracks are complete 
    let unsyncedTracksCount = unsyncedTracks.count 
    var completeCounter = 0 

    syncGroup.enter() 
    for track in unsyncedTracks { 

     registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

      if success { 
       self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
      } 
      else { 
       fail = true 
      } 

      completeCounter = completeCounter + 1 
      if completeCounter == unsyncedTracksCount { 
       syncGroup.leave() 
      } 
     }) 
    } 

    //all requests complete 
    syncGroup.notify(queue: .main) { 
     if fail { 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

我能做到這一點（它也出現工作。 ）？我加了，如果失敗，leave()如果。

var fail = false 
    let syncGroup = DispatchGroup() //used so we can determine if all the register requests have finished 

    //used to make sure notify is only triggered when all registerTracks are complete 
    let unsyncedTracksCount = unsyncedTracks.count 
    var completeCounter = 0 

    syncGroup.enter() 
    for track in unsyncedTracks { 
     if fail { //might save an aync request from getting executed on failure 
      syncGroup.leave() 
      break 
     } 

     registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

      if success { 
       self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
      } 
      else { 
       fail = true 
      } 

      completeCounter = completeCounter + 1 
      if completeCounter == unsyncedTracksCount { 
       syncGroup.leave() 
      } 
     }) 
    } 

    //all requests complete 
    syncGroup.notify(queue: .main) { 
     if fail { 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

Answer 1

那麼，問題是關係到你的fail變量，for循環結構。你一個在循環執行之前將fail指定爲false。當您的代碼進入循環並註冊.enter()時，它會檢查if fail條件，並在fail爲true時調用.leave()。基本上，當其中一個呼叫registerTrack()將fail設置爲true您的代碼處於未確定狀態，因爲registerTrack()是異步的，並且您不知道哪個.enter()調用將標誌設置爲true。 當.enter() count等於.leave() count（例如，兩行失敗）時，CGD決定您的隊列已完成，並且「過早地」調用.notify()。

我建議您刪除標記fail（代碼將按預期工作，但將對所有unsyncedTracks執行registerTrack()）並重新考慮您的解決方案和邏輯。例如，DispatchWorkItem.cancel()可以解決您的問題。